Assessment and Analysis of Al-Zn, Mg, and Mg/Al-Zn Metal-Rich Primers Applied to AA 7075-T651 Under Full Immersion

RIP2025-00066: Age-hardenable aluminum alloys are vital to aerospace applications due to their high specific strength; as such, their corrosion resistance is critical to ensuring long-term sustainable use. To date, the aerospace industry has focused on legacy Al-Mg, Al-Cu-Mg, and Al-Zn-Mg-Cu alloys. These alloys are often prone to intergranular corrosion (IGC) as a result of their alloying composition, aging process, and environmental conditions. One challenge and opportunity is to mitigate this degradation process by imposing sacrificial cathode prevention (SCP). Metal-rich primers (MRP) are a class of active corrosion protection coatings containing sacrificial metallic pigments that are more electrochemically active than the underlying substrate. They inhibit the corrosion of the substrate by providing sacrificial anode-based cathodic protection. Metal-rich primers can provide corrosion protection in three ways, galvanic protection, chemical inhibition, and barrier effect but these depend on the coating used.

In this work, the scanning vibrating electrode technique (SVET) was employed to provide a localized assessment of substrate protection of three MRPs such as the ability to suppress localized corrosion (i.e. pitting) as well as provide broad sacrificial anode-based cathodic protection to AA 70705-T651 substrate exposed at macro-defects or in the case of intact coatings. The coatings investigated in this study were an aluminum-rich primer (AlRP), magnesium-rich primer (MgRP), and a composite magnesium + aluminum-rich primer (MgAlRP) on AA 7075-T651. SVET testing was conducted under full immersion in 1 mM NaCl. The efficacy of these metal-rich primers to provide sacrificial anode-based cathodic prevention was evaluated considering both intact coatings as well as a 1:1 coating/defect area ratio with a macro defect consisting of an unpainted region exposing the substrate. The SVET analysis was complimented with zero resistance ammetery along with DC and AC electrochemistry in 0.6 M NaCl to assess the ability of each MRP to provide protection to the bare AA 7075-T651 substrate. Oxidation characteristics were monitored via scanning electron microscopy (SEM) and elemental mapping via energy dispersive x-ray spectroscopy (EDS) was used to track oxygen signal as a marker for the oxidation of the pigment in each coating system and the substrate. The MgRP and MgAlRP provided superior local corrosion suppression compared to the AlRP. The MgAlRP was determined to be the most effective coating/MRP system for sustained ability to suppress local peak anodic current associated with localized corrosion on bare AA 7075-T651 at coating defects (i.e., protect bare areas).